Intracellular mitochondria synthesize the energy storage molecule ATP in cells using the chemical energy of nutrients through the electron transport system. Enzymes which play an important role in this electron transport system are electron transport system enzymes. When the chemical energy of nutrients is transferred to electrons, high-energy electrons are generated and transported between several enzymes of the electron transport system while the energy of the electrons is released. The released energy is used to form a proton gradient which is used to synthesize ATP. Mitochondria transport protons using the energy of electrons and make the concentration of protons different across the inner membrane. This difference in concentration is called a “proton gradient”. When the proton channel of the inner membrane is open in the state in which the concentration of protons differs between the inside and the outside of the inner membrane, protons flow from an area of high proton concentration to an area of low proton concentration, and the energy of this flow is used to synthesize ATP.
The mitochondrial inner membrane (cristae) includes numerous electron transport system enzymes. When electrons are transported between these electron transport system enzymes, the energy of the electrons is consumed and protons are transported out of the mitochondrial inner membrane. Enzymes located in the mitochondrial inner membrane and the functions thereof will now be described in further detail. The first electron transport enzyme is flavin mononucleotide (FMN). It is an enzyme attached to the mitochondrial inner membrane and receives two electrons and two protons from NADH that is the product of the dehydrogenase reaction. Favin mononucleotide transports the two protons, received from NADH, to the outside of the inner membrane, and transfers the two electrons to coenzyme Q (CoQ) which is the next enzyme of the electron transport system. The coenzyme Q that received the electrons binds to two protons in the inner membrane to form CoQH2, and then transport the two protons out of the inner membrane. At the same time, the two electrons received in the coenzyme are transferred to cytochrome b. The electrons transferred to cytochrome b are transferred sequentially to cytochrome c1, cuytochrome c, cytochrome a, and finally to cytochrome a3 in which the electrons are transferred to oxygen to form water. The protons transported in this process form a proton gradient across the mitochondrial inner membrane.
The electron transport is composed of four different complexes: complex I (NADH: ubiquinone oxidoreductase), complex II (succinate: ubiquinone oxidoreductase), complex III (ubiquinol: cytochrome C oxidoreductase), and complex IV (cytochrome C: oxidoreductase). In the process in which electrons are transferred from complex I through complex IV to an oxygen molecule, hydrogen ions are pumped out of the mitochondrial matrix while a membrane potential is formed. When hydrogen ions flow into the mitochondrial matrix by F0/F1-ATP synthase (complex V), ATP is produced from ADP. This process is referred to as oxidative phosphorylation (OXPHOS).